The present invention relates generally to refractometers for measuring refractive index of a substance, and more particular to an automatic refractometer of compact design for easy hand-held operation.
Refractometers measure the critical angle of total reflection by directing an obliquely incident non-collimated beam of light at a surface-to-surface boundary between a high refractive index prism and the sample to allow a portion of the light to be observed after interaction at the boundary. In transmitted light refractometers, light that is transmitted through the sample and prism is observed, while in reflected light refractometers, the light that is reflected due to total reflection at the surface-to-surface boundary is observed. In either case, an illuminated region is produced over a portion of a detection field of view, and the location of the shadowline between the illuminated region and an adjacent dark region in the detection field of view allows the sample refractive index to be deduced geometrically.
In simpler hand-held refractometers used in industry, a reticle scale is superimposed in the field of view and the operator looks through an eyepiece to observe the location of the shadowline with respect to the reticle scale, which is marked so as to provide desired information such as percentage concentration of solids in the sample. Illumination of the test sample can be provided by ambient illumination, or by a dedicated light source near the sample as disclosed for instance by U.S. Pat. No. 4,650,323. Hand-held refractometers are desirable because they enable periodic on-site measurements to be performed on substances as a means of quality assurance. Since the refractive index of a liquid substance is related to the concentration of elements within the substance, hand-held refractometers are used widely in the soft drink industry to monitor sugar concentration and in the machine tool industry to check the lubricant concentration in cutting fluid. As a further example, U.S. Pat. No. 6,034,762 describes a hand-held refractometer for measuring the water content in hydraulic fluids such as brake fluid. The measurement results of hand-held refractometers have limited accuracy (closeness to the true value) and precision (repeatability regardless of accuracy) due to the fact that an operator judges the shadowline location with respect to a reticle scale, thereby introducing an element of human error into each reading.
The desire for greater accuracy and precision has led to the development of automatic refractometers that remove the guesswork associated with visually determining shadowline location with respect to a reticle scale. U.S. Pat. No. 4,640,616 (Michalik) and U.S. Pat. No. 6,172,746 (Byrne et al.) disclose automatic refractometers wherein a linear scanned array (LSA) of photosensitive elements or xe2x80x9ccellsxe2x80x9d is arranged to detect light after interaction at a sample/prism boundary. In commercial embodiments, the linear scanned array includes a straight line of charge-coupled device (CCD) cells that are scanned electronically to provide a series of pulse signals each having an amplitude proportional to the amount of illumination received by the cell from incident light. Light received by the linear scanned array divides the array into an illuminated region and an adjacent dark region, thereby forming a shadowline on the array. The particular cell or interpolated inter-cell fraction at which the shadowline crosses the linear scanned array is determined by the index of refraction of the sample substance placed in contact with the prism. The output from the photoelectric cells is digitized and processed to find the cell location where the transition shadowline crosses the array, from which the refractive index and concentration of interest can be calculated. Automatic refractometers of the prior art are larger and heavier than their hand-held counterparts, and include optical means designed to provide sufficient light flux initially reaching the detector array to achieve a favorable signal-to-noise ratio with respect to the detector array. In these instruments, compactness in the arrangement of the optical elements is not a limiting design factor.
Therefore, it is an object of the present invention to provide an automatic refractometer that is compact and lightweight for hand-held use.
It is another object of the present invention to provide a hand-held automatic refractometer that provides highly accurate and precise measurement readings.
It is yet another object of the present invention to provide a hand-held automatic refractometer with an optical system that maximizes the use of illumination from a light source to provide a desirable signal-to-noise ratio at a photosensitive array of the refractometer without relying on increased power consumption by the light source.
It is yet another object of the present invention to provide a hand-held automatic refractometer with a physically compact optical system that results in an illumination distribution at a photosensitive array of the refractometer that is free of anomalies that might affect measurement readings.
In furtherance of these and other objects, a hand-held automatic refractometer of the present invention generally comprises a linear scanned array having a plurality of photoelectric cells, and optical means for directing light onto the array such that the particular photoelectric cells of the array which are illuminated by the light are determined by the index of refraction of a sample substance placed in operative association with the optical means, wherein the optical means acts also to receive stray light reflected by the array and redirect the light back onto the array. More specifically, the optical means includes a prism having a sample-receiving surface for establishing a critical angle boundary, a source of non-parallel light obliquely directed at the sample-prism boundary, and a reflective surface positioned proximate the linear scanned array at an acute angle relative thereto. The reflective surface is orientated to perform two functions. First, the reflective surface receives light internally reflected at the sample prism boundary and directs the light toward the linear scanned array. Second, the reflective surface receives a small amount of light that is reflected by the linear scanned array and redirects the light back upon the array. In a preferred embodiment, the reflective surface is made long enough such that a portion of the reflective surface receives none of the initial light flux, but rather is dedicated strictly to returning secondary reflected light from the array back onto the array. Also according to a preferred embodiment, the linear scanned array is arranged to extend in a direction that is parallel to or substantially parallel to the sample-receiving surface for a low-profile design, with the sample surface facing upward and the array facing downward.
The refractometer further comprises signal processing electronics and a display for converting the output from the linear scanned array to a meaningful measurement result and reporting the result.